1. Field of the Invention
The present invention relates to a vehicular lighting unit having a lens provided with aiming pads, utilized when an optical-axis inspection is made, and, more particularly, to an apparatus and method for producing a vehicular lighting unit whose lens is formed by resin-molding.
2. Related Background Art
In the prior art there has been proposed an optical-axis inspection apparatus, for inspecting the optical axis of an automobile headlight, as disclosed in, for example, U.S. Pat. No. 3,520,618. A method for inspecting the optical axis of a headlight comprises the steps of providing three aiming pads protruding from the front surface of the lens of such a headlight, forming a plane, having specific angles with the optical axis of the headlight, by connecting the tips of these aiming pads, and positioning an optical-axis inspection apparatus with respect to the headlight, by utilizing the plane having the specific angles so as to place the apparatus in position with respect to the headlight. This enables the optical axis of the headlight to be inspected properly.
FIG. 6 is a perspective view of a headlight having aiming pads such as those described above. A lens 2 is firmly secured by utilizing a sealing groove 13 formed along the peripheral edge of the front opening 11 of a lighting fixture body 1. A reflector 3, an electric bulb 4, and the like, are contained in a light chamber partitioned with the lighting fixture body 1 and the lens 2. The lens 2 is formed by injection-molding transparent resin into a so-called box lens, including a lens portion 21, whose configuration corresponds to the front opening of the lighting fixture body, a side portion 22, formed around the lens portion, and a sealing leg portion 23, provided at the end edge of the side portion and utilized when the lens portion is fitted to the lighting fixture body. In addition, three aiming pads 25 protrude forward from three places of the surface of the lens portion 21. As shown in a partial exploded perspective view of FIG. 7(a), each of the aiming pad 25 is a protrusion formed by locally increasing the thickness of a part of the lens portion 21.
The headlight is formed by fitting the lens 2, having the three aiming pads 25, to the lighting fixture body 1 and mounting it on an automobile. An optical-axis inspection apparatus 200, shown in FIG. 8, is then set opposite to the front of the headlight, and the leading edge faces of rods 201.about.203, provided in the optical-axis inspection apparatus 200, are each arranged to abut against the three aiming pads 25, so that the optical-axis inspection apparatus 200 may be positioned with respect to the plane specified by the tip of each aiming pad 25. Moreover, the optical-axis inspection apparatus 200 may be positioned in a specific direction with respect to the optical axis O. In this case, two out of the three rods 201.about.203, each abutting against the aiming pads 25, are made movable in the axial direction, whereby the optical-axis inspection apparatus 200 can properly be set opposite to the optical axis of a vehicular lighting unit on various specific planes.
When a lens having aiming pads as described above is made by injection-molding, resin-molding gates are normally designed so that they are placed at the end edge of a sealing leg portion 23. For this reason, when injection molding is conducted, the resin injected from the gates, into a cavity with molds, flows from the gates into various portions to fill up the cavity. Since a portion extending from the sealing leg portion 23 up to the lens portion 21 is substantially uniform in thickness, the resin flows relatively smoothly. However, the cavity in a portion where the aiming pad 25 exists is deeper than the remainder of the cavity, and therefore the flow of the resin in this area tends to become slow and disturbed.
Therefore, there arises a phenomenon in which the disturbance in the flow of resin causes the portion corresponding to the presence of the aiming pad 25 to be surrounded with the resin, resulting in part of the air in the cavity in the portion corresponding to the aiming pad 25 being trapped. When the air trapped therein is forced to escape from the cavity through the resin, the resin undergoes pulsating non-uniformity and causes weld lines W to be produced in the lens portion 21 close to the aiming pad 25 (see FIG. 6). Such weld lines deteriorate not only the external appearance of the lens portion 21, but also the quality of the headlight. Moreover, weld lines that become conspicuous cause light to scatter therein and also deteriorates light distribution characteristics.
Since the aiming pad 25 is made thicker than the lens portion 21, the surface side of the aiming pad 25 cools quickly after the injection molding is terminated, while the inside is subsequently cooled. When the inside of the aiming pad 25 is cooled, its volume shrinks, and as shown in FIG. 7(b), the leading end portion of the aiming pad 25 is deformed in the form of a recess and a so-called "sink mark" H appears. When such a "sink mark" appears, the leading end of the aiming pad 25 recedes by a length of S from the original position, the plane formed with the three aiming pads 25 fails to conform to the designed plane and an accurate optical-axis inspection becomes impossible.